A dead two-metre shark that was found strung up by its tail and slashed across the torso in waters off the NSW central coast was probably still alive when it was targeted, according to a diver who discovered the "horrifying" scene.

Robbert Westerdyk, a member of the Terrigal Underwater Group, had just finished a dive at the HMAS Adelaide shipwreck off Avoca Beach on Saturday morning when he and a group of fellow divers saw a large shadow in the water as they prepared to leave.

I think it still must have had a bit of life and it was still maybe trying to fight

The divers swam over to investigate, and were confronted with a shocking scene: a two-metre, 120-kilogram mako shark suspended on a mooring rope near the shipwreck's bow.

The shark was found near Avoca Beach. Photo: Robbert Westerdyk

The dead shark appeared to have been deliberately tied up, and also had been stabbed on its torso.

"It was really strung up tight by the tail, and then there was a big gash right across the torso of the shark.

The shark was discovered near the wreck of HMAS Adelaide. Photo: Robbert Westerdyk

"We were trying to think of what could have done this. It certainly didn't happen by itself. It's certainly had intervention there by some means.

"It was just a horrifying thing to see.

"I don't even think the shark was fully dead at the time because of the way it was quite gnarled and twisted. I think it still must have had a bit of life and it was still maybe trying to fight."

Mystery: It is not known why this shark was killed and tied up. Photo: Robbert Westerdyk

Mr Westerdyk, who is a keen underwater photographer, took some images of the shark and forwarded them to the NSW Department of Primary Industries, which is investigating the incident.

The divers then cut the shark free and it floated away.

Mr Westerdyk said while it was not illegal to fish for mako sharks, the shark was found within the 200-metre exclusion zone around the HMAS Adelaide.

The shark was slashed and then tied up in the water. Photo: Robbert Westerdyk

"Because it's an artificial reef now, the ex-HMAS Adelaide, it attracts a lot of fish life and of course the fishermen like to go there," he said.

"There is a 200-metre exclusion zone around the area, but you still get the unscrupulous people who want to go in there and poach."

Mr Westerdyk said he had only seen one other mako shark in his many years of diving.

"They are in our waters ... [but] you don't see them regularly at all.

"They're normally a deep ocean fish and they're out there chasing the pelagic fish," he said.

"They usually don't give us any grief at all, but they are a really nice looking fish. They're very sleek and they're very fast."

Glenn Tritton, director of fisheries compliance at the Department of Primary Industries, said the department was working with the RSPCA to try to determine the cause of the shark’s death.

‘‘While it's difficult to speculate on the exact circumstances about these photos, clearly it is disappointing to see these pictures of the mako shark and this type of action is totally unacceptable,’’ he said.